Paint Tool SAI - Very simple to use but offers a lot of functonality just like photoshop, just about 90% of the people i know use this program.

http://www.furaffinity.net/user/soraslipheed/ - Sora uses it alot and produces some top quality stuff.

With Sai you can get a months free trial, then you need to pay (£40 roughly) it is worth it and much cheaper than Photoshop. Its also much easier to get goodwork line with it.

There is Coral Painter which i got for £20something off Amazon. Its has a better painting features than Sai, but i find it harder to draw with.

Lastly there is Gimp 2 http://www.gimp.org/ This is free to download but its not as good as either of the other two.

I just use Photoshop 7. I'm old fashioned like that.